Code of Federal Regulations > Title 40 > Chapter I > Part 50 - PART 50—NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY STANDARDS

U.S. Code of Federal Regulations

Regulations most recently checked for updates: Jan 21, 2026

§ 50.1 - Definitions.

§ 50.2 - Scope.

§ 50.3 - Reference conditions.

§ 50.4 - National primary ambient air quality standards for sulfur oxides (sulfur dioxide).

§ 50.5 - National secondary ambient air quality standard for sulfur oxides (sulfur dioxide).

§ 50.6 - National primary and secondary ambient air quality standards for PM10.

§ 50.7 - National primary and secondary ambient air quality standards for PM2.5.

§ 50.8 - National primary ambient air quality standards for carbon monoxide.

§ 50.9 - National 1-hour primary and secondary ambient air quality standards for ozone.

§ 50.10 - National 8-hour primary and secondary ambient air quality standards for ozone.

§ 50.11 - National primary and secondary ambient air quality standards for oxides of nitrogen (with nitrogen dioxide as the indicator).

§ 50.12 - National primary and secondary ambient air quality standards for lead.

§ 50.13 - National primary and secondary ambient air quality standards for PM2.5.

§ 50.14 - Treatment of air quality monitoring data influenced by exceptional events.

§ 50.15 - National primary and secondary ambient air quality standards for ozone.

§ 50.16 - National primary and secondary ambient air quality standards for lead.

§ 50.17 - National primary ambient air quality standards for sulfur oxides (sulfur dioxide).

§ 50.18 - National primary ambient air quality standards for PM2.5.

§ 50.19 - National primary and secondary ambient air quality standards for ozone.

§ 50.20 - National primary ambient air quality standards for PM2.5.

§ 50.21 - National secondary ambient air quality standards for sulfur oxides (sulfur dioxide).

Appendix Appendix A - Appendix A-1 to Part 50—Reference Measurement Principle and Calibration Procedure for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet Fluorescence Method)

Appendix Appendix A - Appendix A-2 to Part 50—Reference Method for the Determination of Sulfur Dioxide in the Atmosphere (Pararosaniline Method)

Appendix Appendix B - Appendix B to Part 50—Reference Method for the Determination of Suspended Particulate Matter in the Atmosphere (High-Volume Method)

Appendix Appendix C - Appendix C to Part 50—Measurement Principle and Calibration Procedure for the Measurement of Carbon Monoxide in the Atmosphere (Non-Dispersive Infrared Photometry)

Appendix Appendix D - Appendix D to Part 50—Reference Measurement Principle and Calibration Procedure for the Measurement of Ozone in the Atmosphere (Chemiluminescence Method)

Appendix Appendix E - Appendix E to Part 50 [Reserved]

Appendix Appendix F - Appendix F to Part 50—Measurement Principle and Calibration Procedure for the Measurement of Nitrogen Dioxide in the Atmosphere (Gas Phase Chemiluminescence)

Appendix Appendix G - Appendix G to Part 50—Reference Method for the Determination of Lead in Total Suspended Particulate Matter

Appendix Appendix H - Appendix H to Part 50—Interpretation of the 1-Hour Primary and Secondary National Ambient Air Quality Standards for Ozone

Appendix Appendix I - Appendix I to Part 50—Interpretation of the 8-Hour Primary and Secondary National Ambient Air Quality Standards for Ozone

Appendix Appendix J - Appendix J to Part 50—Reference Method for the Determination of Particulate Matter as PM10 in the Atmosphere

Appendix Appendix K - Appendix K to Part 50—Interpretation of the National Ambient Air Quality Standards for Particulate Matter

Appendix Appendix L - Appendix L to Part 50—Reference Method for the Determination of Fine Particulate Matter as PM2.5 in the Atmosphere

Appendix Appendix M - Appendix M to Part 50 [Reserved]

Appendix Appendix N - Appendix N to Part 50—Interpretation of the National Ambient Air Quality Standards for PM2.5

Appendix Appendix O - Appendix O to Part 50—Reference Method for the Determination of Coarse Particulate Matter as PM10-2.5 in the Atmosphere

Appendix Appendix P - Appendix P to Part 50—Interpretation of the Primary and Secondary National Ambient Air Quality Standards for Ozone

Appendix Appendix Q - Appendix Q to Part 50—Reference Method for the Determination of Lead in Particulate Matter as PM10 Collected From Ambient Air

Appendix Appendix R - Appendix R to Part 50—Interpretation of the National Ambient Air Quality Standards for Lead

Appendix Appendix S - Appendix S to Part 50—Interpretation of the Primary National Ambient Air Quality Standards for Oxides of Nitrogen (Nitrogen Dioxide)

Appendix Appendix T - Appendix T to Part 50—Interpretation of the Primary and Secondary National Ambient Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide)

Appendix Appendix U - Appendix U to Part 50—Interpretation of the Primary and Secondary National Ambient Air Quality Standards for Ozone